The present invention relates to visibility detection, and more particularly to detection of airborne visibility impairing or impeding agents. Even more particularly, the present invention relates to improvements in detection of such agents using backscattered infrared radiation (or light).
Fog, heavy rain, sleet, snow, sand and dust are among airborne agents that can cause reduced or impaired visibility on roadways. When present, they are likely to produce unpredictable and dangerous driving conditions. Drivers, perhaps fearing rear-end collisions, blindly proceed into these elements at high speeds. Visibility distance is frequently less than the stopping distance for these vehicles, making it impossible to these drivers to perceive a hazard in time to avoid it by stopping. Every year, news stories abound detailing multiple-vehicle collisions, caused by the "piling-up" of vehicle after vehicle under these dangerous conditions.
Systems have been proposed for detecting fog using an infrared emitter/detector pair for "diffusing" infrared radiation, and for detecting "retrodiffusion" (diffusion back toward the emitter/detector pair). Such systems have been mounted in a vehicle and used to activate and deactivate fog lamps, see U.S. Pat. No. 5,349,267 of Brassier et al., and have been mounted in a wayside (i.e., roadside) housing and used to illuminate flashing warning signs, see "Automatic Fog Alert System Protects Busy Highway," Rural and Urban Roads, Vol. 11 N, No. 5; and "Infrared Device Detects Fog," Public Works, Vol. 104, No. 4. A similar wayside detection system has been used to illuminate lights marking outside edges of pavement. See "Airport Lights Shine on Fog-Bound Highway," American City and County, Vol. 95, No. 6. In another system, infrared lasers and detectors mounted in a vehicle are used to detect a visibility range by measuring backscatter. See "Infrared Lasers Prevent Crashes: The Anti-collision Fog System Makes for Safer Driving," NEW-TECH NEWS, No. 1-19.
Unfortunately, the above-described wayside detection systems fail to address several important problems. For example, in the event the field of vision of the emitter or detector becomes impaired due to the buildup of dust, and/or "road dirt," such as brake pad particles, tire particles or airborne oil particles, on an emitter lens or detector lens, the detector may indicate that fog does not exist (i.e., that there is little or no backscattered infrared radiation detected), when in fact visibility is fog-impaired (i.e., there is significant backscattered infrared radiation). One potential solution that may be adaptable to a wayside system is proposed in the NEW-TECH article, cited above, which describes the use of a wiper, similar to a windshield wiper. Problematically however, even if the wiper could be adapted for use in a wayside system, it still does not assure that the field of view is not impaired--rather it only attempts to mechanically remove any impairment that might be impairing the field of view. Furthermore, in that the use of a wiper implies the use of motor to turn the wiper, an undesireably high current is required to operate the wiper. While such current is readily available in an automobile, it may not be available in a wayside system, which preferably could be powered by a solar-recharged battery.
The above mentioned heretofore known visibility detectors rely on the fact that fog and other agents reflect, retrodiffuse or backscatter infrared light directed into a fog bank or other area of high visibility inhibiting agent concentration.. Unfortunately, fog and other airborne visibility reducing agents are not of a constant density, i.e., they may be concentrated in pockets, may billow and may be affected by gusts of wind. As a result, the density of the fog, or other visibility reducing agent, may vary locally even though visibility overall remains low. The above-described visibility detection systems, as described in the above-cited documents, fail to account for this quality, and as a result are subject to falsely or prematurely detecting that visibility has been restored, or that visibility has been reduced, in response to pockets of visibility or pockets of, e.g., fog. Thus, the lights controlled by the above-mentioned wayside systems will be controlled by the emitter/detector pair to illuminate and extinguish as pockets of clear and pockets of, e.g., fog, pass over the emitter/detector pair. Such will occur even though the roadway may be predominantly clear or, e.g., predominantly foggy.
One solution to the problem of false detection is described in the '267 patent, wherein in order to prevent inappropriate illumination or extinction of a fog lamp, a time delay module is arranged on the output of a comparator. As a result, the emitter/detector pair of the '267 patent must detect adequate visibility or inadequate visibility for a predetermined time period before signaling an indication of such to circuity controlling the illumination and extinction of the foglamps. Unfortunately, if adequate visibility has been restored (after a period of inadequate visibility), but small pockets of poor visibility continue to pass over the emitter/detector pair; or if visibility has become inadequate (after a period of adequate visibility), but pockets of clear continue to pass over the emitter/detector pair, the visibility detector of the '267 patent will continue to signal an inappropriate illumination or extinction, so long as such pockets pass over the emitter/transmitter pair at an interval shorter than the prescribed time delay. If the prescribed time delay is shortened to avoid this prolonged erroneous illumination or extinction, the emitter/detector pair may signal an illumination every time a pocket of impaired visibility passes, or an extinction every time a pocket of clear passes, so long as such pockets are large enough to increase or reduce backscatter for the shortened time delay.
A further problem with the above-mentioned heretofore known visibility detectors, as described in the above-cited references, is calibration. In the '267 patent, for example, the visibility detector is calibrated either using a potentiometer to adjust a reference threshold and/or using a secondary detector to a detect degree of pollution, atmospheric pressure, and ambient temperature. Unfortunately, such a calibration scheme, if it could be used with a wayside visibility detector, requires individual manual adjustment at each emitter/detector at the time of installation, so as to account for environmental retrodiffusion or backscatter as a result of, e.g., trees, guard rails, road signs and the like. Such manual adjustment would require that more highly skilled personnel be utilized for installation, and, because such environmental backscatter changes over time, because, for example, trees change shape and size (and therefore amount of backscatter) as they grow, periodic manual adjustment of such a wayside visibility detector unfortunately would be necessary.
One major problem not recognized in any of the above-mentioned references is meniscus conduction of infrared radiation from the emitter to the transmitter. Meniscus conduction occurs when a layer of water adheres to the surface of the emitter or emitter lens, the detector or detector lens, and a housing surface interposed between them. When infrared light is emitted from the emitter, some of it is diffused away from the emitter. However, when the layer of water (i.e., meniscus) is present, some of the emitted infrared light is reflected or diffused by the surface of the layer of water and is conducted by the water away from the emitter across the interposed surface of the housing to the detector. Problematically, such conducted infrared light is interpreted by the detector to be backscattered infrared light, and therefore increases the detected amount of backscatter. In turn, as the detected amount of backscatter increases, the visibility is incorrectly determined by the visibility detector circuitry to have decreased. As a result, when such meniscus conduction occurs, such as may occur during a light rain or mist, or when wayside water sprinklers are activated, the visibility detectors, as described above and in some of the cited references, are prone to indicate that visibility is impaired, when it may not be.
Thus, significant needs in visibility detectors, and in particular in wayside visibility detectors, remain unaddressed by heretofore known visibility detectors. The present invention advantageously addresses the above and other needs.